As far as I can recall, earlier this year it was just a normal URL-fragment-like string (starting with #), without the exclamation mark. But now it's a shebang or hashbang (#!), which I've previously only seen in shell scripts and Perl scripts.

The new Twitter URLs now also feature the #! symbols. A Twitter profile URL, for example, now looks like this:

http://twitter.com/#!/BoltClock

Does #! now play some special role in URLs, like for a certain Ajax framework or something since the new Facebook and Twitter interfaces are now largely Ajaxified? Would using this in my URLs benefit my Web application in any way?

Which is why I'm puzzled as to what it's doing in a Facebook URL.
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BoltClock♦Jun 9 '10 at 19:59

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FWIW, it's not just shell and perl scripts, but any script run on a unix like system. The #! line tells the shell what the interpreter for that script is... of course, my comment has nothing to do with facebook or twitter
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bluesmoonOct 16 '10 at 22:57

6 Answers
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This technique has mostly been supplanted by the ability to use the JavaScript History API that was introduced alongside HTML5. For a URL like www.example.com/ajax.html#!key=value, Google will check the URL www.example.com/ajax.html?_escaped_fragment_=key=value to fetch a non-AJAX version of the contents.

Are you sure that is all there is to it? I often find that the page loading hangs on a shebang URL on facebook (even after many reloads), but if you manually remove the #!, it works. Not to mention you often get "1.5 URLs" (i.e. the old URL remains, and just has the new part added to it (i.e. photo.php?id=... twice, but with different ids). Not to mention that "#!" is also added to facebook-mail URLs, which probably aren't (and shouldn't be) indexable. In any case I find the shebang extremely annoying since it seems to be the reason for so many page faults on my slow home line.
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PederyOct 15 '10 at 3:15

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That Facebook has bugs doesn't make those bugs the fault of two characters in the URL. If the site is coded properly to understand and generate them, crawlable AJAX URLs are quite handy. Lots of other things on Facebook glitch out, too.
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ceejayozOct 15 '10 at 3:19

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@Pedery: I have only ever seen that issue with Facebook. I agree, it drives me up the (non-Facebook) wall all the time.
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BoltClock♦Oct 15 '10 at 3:22

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As for search engines, having an indexable AJAX URL doesn't make the page get indexed anymore than having an indexable non AJAX URL does. Facebook uses this URL format for more than just Google's benefit - it also makes pages accessed via AJAX on Facebook bookmarkable when they otherwise wouldn't be.
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ceejayozOct 15 '10 at 3:24

To have a good follow-up about all this, Twitter - one of the pioneers of hashbang URL's and single-page-interface - admitted that the hashbang system was slow in the long run and that they have actually started reversing the decision and returning to old-school links.

I always assumed the ! just indicated that the hash fragment that followed corresponded to a URL, with ! taking the place of the site root or domain. It could be anything, in theory, but it seems the Google AJAX Crawling API likes it this way.

The hash, of course, just indicates that no real page reload is occurring, so yes, it’s for AJAX purposes. Edit: Raganwald does a lovely job explaining this in more detail.

I would be very careful if you are considering adopting this hashbang convention.

Once you hashbang, you can’t go back. This is probably the stickiest issue. Ben’s post put forward the point that when pushState is more widely adopted then we can leave hashbangs behind and return to traditional URLs. Well, fact is, you can’t. Earlier I stated that URLs are forever, they get indexed and archived and generally kept around. To add to that, cool URLs don’t change. We don’t want to disconnect ourselves from all the valuable links to our content. If you’ve implemented hashbang URLs at any point then want to change them without breaking links the only way you can do it is by running some JavaScript on the root document of your domain. Forever. It’s in no way temporary, you are stuck with it.

I think your criticism of hashbangs is valid, but using just pushState as a substitute means that we would lose the ability to load content within a single page app based on the URL. So then URLs can't be shared.
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LukeJul 23 '14 at 22:30

The octothorphe/number-sign/hashmark has a special significance in an URL, it normally identifies the name of a section of a document. The precise term is that the text following the hash is the anchor portion of an URL. If you use Wikipedia, you will see that most pages have a table of contents and you can jump to sections within the document with an anchor, such as:

https://secure.wikimedia.org/wikipedia/en/wiki/Alan_Turing identifies the page and Early_computers_and_the_Turing_test is the anchor. The reason that Facebook and other Javascript-driven applications (like my own Wood & Stones) use anchors is that they want to make pages bookmarkable (as suggested by a comment on that answer) or support the back button without reloading the entire page from the server.

In order to support bookmarking and the back button, you need to change the URL. However, if you change the page portion (with something like window.location = 'http://raganwald.com';) to a different URL or without specifying an anchor, the browser will load the entire page from the URL. Try this in Firebug or Safari's Javascript console. Load http://minimal-github.gilesb.com/raganwald. Now in the Javascript console, type:

Still no refresh. Use the back button to see that these URLs are in the browser history. The browser notices that we are on the same page but just changing the anchor, so it doesn't reload. Thanks to this behaviour, we can have a single Javascript application that appears to the browser to be on one 'page' but to have many bookmarkable sections that respect the back button. The application must change the anchor when a user enters different 'states', and likewise if a user uses the back button or a bookmark or a link to load the application with an anchor included, the application must restore the appropriate state.

So there you have it: Anchors provide Javascript programmers with a mechanism for making bookmarkable, indexable, and back-button-friendly applications. This technique has a name: It is a Single Page Interface.

p.s. There is a fourth benefit to this technique: Loading page content through AJAX and then injecting it into the current DOM can be much faster than loading a new page. In addition to the speed increase, further tricks like loading certain portions in the background can be performed under the programmer's control.

p.p.s. Given all of that, the 'bang' or exclamation mark is a further hint to Google's web crawler that the exact same page can be loaded from the server at a slightly different URL. See Ajax Crawling. Another technique is to make each link point to a server-accessible URL and then use unobtrusive Javascript to change it into an SPI with an anchor.

@imaginonic: I'm late, but as perfectly crafted as it is, 90% of it doesn't touch on the #! aspect of my question at all. That's why he said it's redundant. The number of upvotes here is likely due to the high traffic when my question made it to Hacker News coupled with the sheer length alone of this answer.
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BoltClock♦Feb 21 '12 at 18:58

First of all: I'm the author of the The Single Page Interface Manifesto cited by raganwald

As raganwald has explained very well, the most important aspect of the Single Page Interface (SPI) approach used in FaceBook and Twitter is the use of hash # in URLs

The character ! is added only for Google purposes, this notation is a Google "standard" for crawling web sites intensive on AJAX (in the extreme Single Page Interface web sites). When Google's crawler finds an URL with #! it knows that an alternative conventional URL exists providing the same page "state" but in this case on load time.

In spite of #! combination is very interesting for SEO, is only supported by Google (as far I know), with some JavaScript tricks you can build SPI web sites SEO compatible for any web crawler (Yahoo, Bing...).

The SPI Manifesto and demos do not use Google's format of ! in hashes, this notation could be easily added and SPI crawling could be even easier (UPDATE: now ! notation is used and remains compatible with other search engines).

Take a look to this tutorial, is an example of a simple ItsNat SPI site but you can pick some ideas for other frameworks, this example is SEO compatible for any web crawler.

The hard problem is to generate any (or selected) "AJAX page state" as plain HTML for SEO, in ItsNat is very easy and automatic, the same site is in the same time SPI or page based for SEO (or when JavaScript is disabled for accessibility). With other web frameworks you can ever follow the double site approach, one site is SPI based and another page based for SEO, for instance Twitter uses this "double site" technique.

What about progressive enhancement principle? Website shouldn't crash of fail due to disabled JavaScript. And trust me, javascript is disabled not just in outdated browsers but also by many security aware users who do not like executing random JS.
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Roman RoyterMar 29 '11 at 4:05